The Miami Heat embarked on Game 2 of the NBA Finals burdened with the most overused term in playoff basketball. It was a “must-win” showdown.

Not true, of course, in a best-of-7 series, but Miami’s players acted like it was, counterpunching the San Antonio Spurs 103-84 and outfitting themselves for the venture into hostile territory.

Sunday’s performance before a relieved AmericanAirlines Arena crowd went beyond assertive. It was flamboyant.

It was over midway through the fourth quarter, when Spurs coach Gregg Popovich made his starters watch the rout and stew, stone-faced, from the bench.

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Losing Game 1s might become a goal of the Heat. It continued a pattern of bounding out of holes when the players feels a few particles of dirt shoveled onto their faces.

The Heat got this one going in the backcourt. The evidence of the Heat’s preferred fast and furious pace was on the players’ faces when they leaned on their knees during breaks, sweat dripping from foreheads like they were leaky faucets.

After being burned by Tony Parker in the fourth quarter of Game 1, the Heat clamped down on the fleet Frenchman, limited him to 13 points and hassled him into five turnovers. He took one shot and scored zero points in the fourth.

This time, it was Miami’s point guard, the frequently maligned Mario Chalmers, who was the key, leading all scorers with 19 points. On Sunday, Chalmers wasn’t merely tagging along with the Big 3, who often treat him like a pesky little brother. He was the maestro, conducting the show as the best point guards do.

Instead of flitting around the periphery, Chalmers lowered his head and assailed the lane, driving for baskets or braking for jumpers. At the end of the third quarter, as the Heat enlarged a two-point edge, Chalmers wheeled by Parker and Kawhi Leonard for a swooping bank shot plus a free throw to make it a 10-point lead.

Chalmers also played like an indispensable floor general, dishing to Ray Allen for a three-pointer and niftily bounce-passing to LeBron James for a layup on successive possessions.

Chalmers reprised his role from last year’s Finals, when he rose to the challenge against Oklahoma City and raised his scoring average by three points.

“You can’t teach that quality — big-game guts,” Heat coach Erik Spoelstra said of Chalmers. “They feel most alive in these situations when you typically would feel the most pressure. He drives me crazy in January and December, but this time of year you like to see it. He’s got incredible confidence in his game. He’s shown that — I wouldn’t say irrational — but you’ve got to have guts to play with our guys or you get swallowed up.”

Chalmers and those assigned to neutralize Parker maneuvered under screens to contain one of the game’s best screen splitters.

“Rio has to play big in multiple facets,” James said. “He’s guarding the best point guard in the league, but he also has to make Tony work on the defensive end. We started to get a little flow. I told him to keep attacking and go for the kill.”

Dwyane Wade, too, was more aggressive and moved fluidly in close quarters once again. He was whirling and dipping in the paint, kicking the ball out to Chalmers for a three-pointer on one play, sneaking along the baseline and springing for a shot put into the basket on the next.

Wade, who contributed 10 points and six assists, had his turns on Parker, as did Norris Cole. They wouldn’t let Parker turn the corner with his usual panache. The Spurs made 16 turnovers after committing only four in Game 1.

“They were really aggressive on the pick-and-rolls, and I just have to take better care of the basketball,” Parker said. “If we can keep our turnovers under 10, that would be good because when we turn it over it’s a fast break for them.”

Manu Ginobili made uncharacteristic fumbles and has yet to hit his stride.

“They had better hands, were more active, made us run more,” he said. “Our rotations were late. It was Miami who turned it on.”

Danny Green was a problem for Miami. He nailed all six of his shots, and his five three-pointers made without a miss set a Finals record. and was crushing the Heat each time it tried to pull away. He blocked a James shot and slid by James with a baseline move. But he, too, went silent in a brief fourth quarter, pulled after the Heat went ahead by 24 points and the game caved in.

“They always tell me to be more aggressive and be more of a leader and that’s what I’m trying to do,” Chalmers said.

The Heat’s shifting matchups threw a wrench into the Spurs’ machine. Heat defense generated offense, which has been and has to be the core game plan for Miami to “play to its identity,” as Spoelstra likes to say. No team is going to outthink the Spurs, who know each other so well and adapt to Popovich’s stratagems with calm confidence.

It will be a long week in San Antonio. Miami’s guards must keep dictating the narrative if they want to win a second straight title. Wade can’t fade out. Miami’s game flows from its backcourt. In Game 2, Chalmers made big waves.